if

Rust’s take on if`ifis not particularly complex, but it’s much more like theif` you’ll find in a dynamically typed language than in a more traditional systems language. So let’s talk about it, to make sure you grasp the nuances.

if`if` is a specific form of a more general concept, the ‘branch’. The name comes from a branch in a tree: a decision point, where depending on a choice, multiple paths can be taken.

In the case of if`if`, there is one choice that leads down two paths:

fn main() { let x = 5; if x == 5 { println!("x is five!"); } }

let x = 5;

if x == 5 {
    println!("x is five!");
}

If we changed the value of x`xto something else, this line would not print. More specifically, if the expression after theifevaluates totrue, then the block is executed. If it’sfalse`, then it is not.

If you want something to happen in the false`falsecase, use anelse`:

fn main() { let x = 5; if x == 5 { println!("x is five!"); } else { println!("x is not five :("); } }

let x = 5;

if x == 5 {
    println!("x is five!");
} else {
    println!("x is not five :(");
}

If there is more than one case, use an else if`else if`:

fn main() { let x = 5; if x == 5 { println!("x is five!"); } else if x == 6 { println!("x is six!"); } else { println!("x is not five or six :("); } }

let x = 5;

if x == 5 {
    println!("x is five!");
} else if x == 6 {
    println!("x is six!");
} else {
    println!("x is not five or six :(");
}

This is all pretty standard. However, you can also do this:

fn main() { let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32 }

let x = 5;

let y = if x == 5 {
    10
} else {
    15
}; // y: i32

Which we can (and probably should) write like this:

fn main() { let x = 5; let y = if x == 5 { 10 } else { 15 }; // y: i32 }

let x = 5;

let y = if x == 5 { 10 } else { 15 }; // y: i32

This works because if`ifis an expression. The value of the expression is the value of the last expression in whichever branch was chosen. Anifwithout anelsealways results in()` as the value.